The present invention relates to a driving mechanism for a vacuum circuit breaker, and more particularly to a driving mechanism of a vacuum circuit breaker which is frequently operated such as that used in an on-load tap changer.
As is well known, a vacuum circuit breaker is provided with bellows for permitting linear reciprocating motion of the movable contact rod while maintaining a hermetic seal. Conventional driving mechanisms drive the movable contact rod at predetermined velocities for opening and closing the circuit breaker.
However, through repeated interruptions of current, the movable and the stationary contacts of the breaker are consumed and worn out due to arcs across the contacts. For instance, a vacuum circuit breaker for an on-load tap changer is required to withstand several millions of repeated interruptions before the total wear of the movable and stationary contacts becomes about 2 mm when the vacuum circuit breaker has to be replaced. As the movable and the stationary contacts are worn out, the movable contact has to be advanced further in order to engage with the stationary contact. The position at which the movable contact rests (the retracted position of the movable contact) remains fixed. As a result, the travel or stroke of the movable contact is increased, and accordingly the amplitude of variation in length of the bellows is increased. With increased amplitude, the fatigue of the bellows in accelerated.
FIG. 1 shows a relation between the total wear .delta. of the movable and the stationary contacts and the distance S from the retracted position of the movable contact and the advanced position at which the movable contact engages with the stationary contact. In a conventional mechanism where the retracted position is fixed, the distance S coincides with the travel or the stroke of the movable contact and hence with the amplitude of variation in the length of the bellows. The initial stroke S.sub.1 (when the contacts are not yet worn) is determined according to the voltage across the contacts and the required interrupting capacity.
As openings and closings of the vacuum circuit breaker are repeated, wear of the contact is gradually increased and the stroke S of the movable contact is increased as depicted by dashed line S. When the wear becomes .delta..sub.2, the stroke S.sub.2 of the bellows is given by: EQU S.sub.2 =S.sub.1 +.delta..sub.2
Accordingly, the amplitude of variation in length of the bellows is increased, the fatigue of the bellows is accelerated and the service life of the bellows is shortened.